Your argument bears zero connection to the idea that Mark Sanchez-level production would have this team in contention 13 games out of 16.
Mark Sanchez caliber players can be had in free agency and trades. If you're going to spend a 1st round pick on a player, it's not someone you want to rent for a few years hoping to upgrade him A.S.A.P.
Frankly, there will be QBs in the 2nd and 3rd round who are better than what we've got and there will be FA QBs who end up as backups elsewhere who are better. All good reasons to not draft a QB in the 1st round simply because we need a QB.